(1) Field of the Invention
This invention relates to a road heater used for making warm enough to melt and remove snow on the road.
(2) Description of the Related Art
Heretofore, a warm water-circulating system and an underground water spraying system have been widely employed for melting snow on the road or in other places. In recent years, an electrical road-heating system has become popular because of ease in laying and maintenance.
The heating element used in the electrical road-heating system is a linear heating element comprising a nichrome wire or a carbon heating element, which is covered with a thermal-resistant vinyl chloride resin insulator (Japanese Unexamined Patent Publication No. 49-114232 and Japanese Unexamined Utility Model Publication No. 63-65704). The linear heating element has problems such that the covering insulator thereof has a poor thermal resistance and a poor flexibility, and, when stressed in the radial direction, it is liable to be distorted in the radial direction.
These problems lead to the following disadvantages.
(i) To avoid damage of the covering insulator of the heating element, an asphalt mortar composition which is free from crushed stones must be laid instead of an asphalt concrete, composition which is a mixture of asphalt and crushed stones. However, the strength of the road surface as paved with the asphalt mortar composition is low as compared with the strength of that as paved with the asphalt concrete composition.
(ii) When an asphalt mortar composition is used, the pavement of roadbed must be conducted after the asphalt mortar composition is allowed to cool to about 120.degree. C. This is time-consuming. Further, as the laid-asphalt mortar composition is pressed and smoothed by a roller at a relatively low temperature, it is difficult to strengthen the paved roadbed to the desired extent.
(iii) A substantially long time is required for pressing and smoothing the asphalt mortar composition laid on the roadbed because the asphalt mortar composition is rolled at a relatively low temperature. Thus the total time required for the completion of pavement is very long, which spans from the cooling of the hot asphalt mortar composition to the completion of pressing and smoothing the laid-asphalt mortar composition.